Method of tightening high strength bolts and apparatus for use therein



METHOD OF TIGHTEIiINZ'i HIGH STRENGTH BOLTS AND APPARATUS FOR USE THEREIN Filed Jan. 18. 1966 2 Sheets-Sheet 1 a: K o N a 1 o r :2 :\\-b N N g T 4 I MVem-OR:

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METHOD OF TIGHTENING HIGH STRENGTH BOLTS AND APPARATUS FOR USE THEREIN Filed Jan. 18. 1966 2 Sheets-Sheet 2 Fig. 30

'iwwejumilLL United States Patent METHOD OF TIGHTENING HIGH STRENGTH BOLTS AND APPARATUS FOR USE THEREIN Peter J. Gill, Codsall, Wolverhampton, England, assignor to G.K.N. Group Service Limited, London, England, a

British company Filed Jan. 18, 1966, Ser. No. 521,377 5 Claims. (Cl. ill- 52.3)

ABSTRACT OF THE DISCLOSURE Apparatus for tightening a nut upon a bolt and giving a visual indication that tightening has achieved a predetermined tension in the bolt shank, comprising an impact wrench in which a motor applies torque to a nut engaging socket through a disengageable drive which includes a hammer device connected to the motor and an anvil device connected to the socket, and in combination therewith a marking tool connected to the anvil device for rotation therewith and also being slidable axially in relation to the nut engaging socket and means for moving said marking tool into engagement with the shank of the bolt to make a mark thereon, said means being operated by a relative movement between the hammer device and the anvil device when the wrench is impacting.

This invention relates to a method of tightening a nut upon a bolt in a high strength bolt assembly and includes a method of testing to ascertain if the correct tightening of the nut and bolt assembly has been achieved. The invention also relates to apparatus for use in the method.

High strength bolts are currently replacing rivets to a large extent in structural work and it is recognized that to achieve maximum efficiency of bolt usage the nut must be tightened down on the bolt to such an extent as to achieve, at least, a predetermined minimum tension in the bolt shank. This minimum may be exceeded but the specified minimum tension must be reached or passed for best results.

One method which has been employed to ensure the attainment of the predetermined tension or pre-load in the bolt shank is a method which involves determining the torque which is required to achieve the desired tension and then, having determined such torque for a particular size of bolt, each nut is tightened to this predetermined torque.

The drawback attached to this method, however, is that bolts of the same size may vary in thread form or in condition of the thread, and this will affect the relationship between applied torque and tension in the bolt shank, so that there is no certain guarantee that bolts which have been tightened to the same torque will all have the same tension in their shanks.

At present, another method of achieving the desired tension in the bolt shank is favoured by constructors and this is known as the turn of the nut method. This method relies upon the fact that once the nut has engaged the work and is solid against the work further rotation of the nut results in stretching of the bolt shank. Reference to the work here will be understood as meaning the members, such as plates, which are being clamped together by the bolt and nut assembly, and the surface of one of these plates (or a washer under the nut) will constitute an abutment which is engaged by the face of the nut after the nut has been run on to the bolt thread and the nut is spoken of as being solid when it engages this abutment and therefore encounters resistance to further axial movement along the bolt shank. (This state is also sometimes referred to as being the state when first grip is obtained.) The stretch of the bolt shank which is "ice achieved by further rotation after the nut has become solid (or first grip) is related to the tension in the shank, so that for a certain rotation of the nut around the aXiS of the shank the desired tension can be achieved in the bolt shank and this can be predetermined for any particular size and type of bolt.

There is a variation of this turn of the nut method, which is favoured by some engineers because it reduces uncertainties in the operators judgement of the occurrence of first grip. In this variation of the method a certain torque is applied to the nut, sufficient to take it Well beyond the first grip stage, and then the further measured amount of nut rotation is applied to ensure attainment of the desired tension in the bolt shank.

Hereinafter the expression datum position is used to define the position from which the measured amount of nut rotation is made; this is either the position of first grip, when the nut becomes solid (in the one case), or, the position reached after the application of the certain torque to take the nut past first grip (in the other case).

The present invention is concerned with this turn of the nut method in both the forms referred to above. Hitherto, when using such method there: has had to be a visual observation of the angular movement of the part of the wrench which is engaging the nut. This part is generally a socket and the operator has to observe the amount of rotation of the socket occurring from the datum position. This is generally obtained by having a mark on the nut-engaging socket but there is this drawback that the success of the method depends upon the human factor of the operator correctly observing the desired amount of angular rotation of the socket with reference to this marking, and there is no reliable means for subsequent checking and inspection.

Alternatively, it is possible to operate the method by marking both the nut and abutment surface at the datum position and then, after applying the extra turn to the nut, again marking the position reached. Whilst this is somewhat more satisfactory, it still entails the human factor of the operator marking the nut and abutment surface correctly.

It is an object of the present invention to provide a method of tightening a high strength bolt assembly, using the turn of the nut principle, and a method also of testing to ensure that the correct amount of nut rotation has been obtained, and which method will eliminate as far as possible, any possible error from the human factor.

Another object of the invention is the provision of an impact wrench for use in carrying out the method of tightening and testing high strength bolt assemblies.

A further object of the invention is to provide an improved form of impact wrench which will provide a visual and permanent marking to indicate the angular rotation of the nut when using the wrench for tightening a bolt with the turn of the nut method.

An impact wrench is herein defined as being the type of wrench in which a motor applies torque to a nutengaging socket through a disengageable drive which includes a hammer device connected to the motor and an anvil device connected to the socket. When the nut is being rotated the hammer device engages the anvil device and there is a direct drive until a predetermined torque resistance is encountered, once this position is reached the reaction causes the hammer device to be thrown out of engagement and the motor accelerates so that the hammer device next engages and strikes the anvil with substantial force and causes the nut to turn. The operation is then repeated with the hammer device alternatively being thrown out of engagement and re-engaged to drive the anvil device, and when this is happening the wrench is referred to as being in the impacting condition.

In. broad. terms the method according .to the invention involves the use of a marking tool which applies a mark to the end of the bolt to give a visual indication of the angular rotation of the nut after the datum position, and the improved wrench embodies a marking tool which moves in an arcuate path about the axis of the bolt, together with the member of the wrench which applies torque .to the nut.

However, with such an arrangement, the operator may not apply enough angular rotation of the nut to achieve the desired tension and when subsequently inspected the mark on the bolt will indicate that further rotation of the nut is'required. It is then necessary to be able to reengage the wrench with the nut in such manner that the marking tool will continue to apply a mark to the end of the bolt, starting from the end of the previously made mark, so that the final inspection can tell accurately if the extra amount of rotation has been made.

A still further object of the present invention is to provide a nut and socket combination which will enable this re-engagement of the wrench for extra tightening to. be done so that the marking tool accurately registers with the end of the previously made mark.

The invention is illustrated in the accompanying drawings and described hereinafter, by way of example, with reference to such drawings in which:

FIGURE 1 is a section through part of an improved impact wrench used in the performance of the method.

FIGURE 2 is a section on line 22 of FIGURE 1.

FIGURE 3 is a side view and end views of a socket for use with the wrench according to the invention.

The invention is illustrated in one form in the accompanying drawings, FIGURE 1 of which shows part of an impact wrench embodying the invention. It will be appreciated that the method according to the invention may be carried out with all type of impact wrenches and other power operated wrenches, and the following description therefore is to be taken as being by way of example only. With reference to the accompanying drawing, as the general construction and operation of an impact wrench is well known, only the one end part of the wrench is shown, this being the forward end having a fixed enclosing casing which houses the hammer and anvil devices.

The rotary drive from the pneumatic motor, which is in the known part of the wrench (not shown), is connected to the cylindrical hammer device 11 by means of a square section shaft engaging in the square socket 12 provided in one end of the cylindrical driving sleeve 13 which works within the central bore of the hammer device, the hammer device 11 being in the form of a hollow sleeve provided at its forward end 14 with a number of forwardly projecting dogs 15. The driving connection between the driving sleeve 13 and the hammer device is by means of balls 16 working in helically inclined tracks 17 on the outer surface of the sleeve 13 and also working in a track in the interior of the hammer device 11.

A strong coil compression spring 18 acts between the forward end 14 of the hammer device and the rearward end of sleeve 13 internally thereof so as normally to urge the dogs into engagement with co-operating dogs 19 formed on the rearward end of the anvil device which further comprises the sleeve 20 rotatably mounted in a hearing sleeve 21 in the forward end of the body 10.

As so far described, the wrench is of generally known form and operates in known manner with the dogs 15 being thrown out of engagement with the dogs 19 when a certain torque resistance to turning of the nut is encountered resulting in axial movement of the hammer device 11 away from the anvil device 20 and compression of the spring 18, and the dogs 15 being urged back into engagement by the spring 18 when the driving motor has accelerated, to give impact to the anvil device and thus turn the nut. The forward end of the anvil device at 22 is formed in such manner as to have non-rotatable engagement with a socket such socket being provided with a nut- 4 engagingportion so that rotationof the anvil device 20 causes rotation of the nut. One form of socket is shown in FIGURE 3.

In accordance with the invention, the anvil device 20 incorporates a marking tool which takes the form of a shaft 23 of square cross-section mounted slidably in a central square section axial hole in the forward endof the anvil device and at its rear end this shaft23 is connected to a piston 24 working slidably in a cylinder 25 formed co-axially within the body of the anvildevic'e.

The extreme end of the shaft 23 is of chisel form so that its operative point 26 is offset from the axis 27 of rotation of the nut and the shaft 23 is retained in the anvil device by means of the screw 28 which engages a slotted part 29 in the shaft but permits limited axial sliding movement of the shaft 23 Within the end 22 of the anvil device. I I

Within the cylinder 25 the piston 24 is acted upon by a spring 30 which normally urges the piston and shaft 23 rearwardly and the end of the cylinder 25 is provided with a valve seating 31, the opening through which is normally closed by the valve member 32 pressed onto the seating by a coil spring 33, one end of which engages the valve member 32 and the other end of which bears upon a flange 34 on a screwed adjusting stud 35 mounted in the end of the cup member 36 which is itself fixed in the/forward end of the hammer device 11.

The strength of the spring 18, and thus the magnitude of its resisting force, will depend upon which of the two forms of the turn of nut method is being operated. In one form the spring 18 will be compressed by the reaction which is encounered as soon as the nut becomes solid whereas in the other case the spring 18 will be stronger and will not be compressed until the nut has gone past first grip and a predetermined amount of torque resistance is encountered. This is the datum position when the wrench commences to impact.

As shown in the drawing, the parts are in the position where the dogs 15 of the hammer device are in engagement with the dogs 19 of the anvil device, and in this condition, with the motor running and the socket on the end of the anvil device engaging a nut, the nut is turned continuously until the datum position is reached. During such turning of the nut the valve member 32 remains closed. The pneumatic circuit of the impact wrench is modified so as to by-pass a certain amount of air under pressure from the normal supply to the motor into the interior of the casing 10, as indicated by arrow A, and when the valve member 32 is opened this air under pressure can enter the cylinder 25. I

As soonas the datum position is reached and-the wrench commences to impact the rearward axial movement of the hammer device 11 will result in the valve member 32 being lifted of its seating so that air under pressure enters the cylinder 25, forcing the piston 24 forwardly against the pressure of its return spring,.thus forcing the shaft 23 forwardly .to cause its pointed end 26 to bite into the end face 40'of the bolt shank where it protrudes beyond the nut 41 which is being tightened upon, and so long as the impacting is repeated then the forward movement of the'shaft 23 will be repeated and a succession of marks will be made on the end face of the bolt shank. v

In practice, such marks result in the formation ofa scratch or groove on the end of the bolt shank which is in the form of an are about the axis 27, and thisarc can be checked for correct length to determine whether or not the correct amount of turn has been applied to the nut.

' In the above example, the marking device is applied to the end face of the bolt shank so as to mark an arc by scratching or cutting a slight groove in this end face the groove being made up of a series of impressions each of which is made everytime the wrenchimpacts. In ariother aspect the marking device may make marks such assaezs as depressions at the beginning and at the end of the turn of the nut movement or it may be adapted to make a series of such marks defining a dotted line are.

Other forms of marking may be employed but all are adapted to provide a visual indication on the bolt shank of the angular amount of turn of the nut and also a permanent marking which can be subsequently inspected and checked.

The method according to the invention may be carried out by using any type of power-operated wrench which automatically changes its mode of operation when a predetermined torque resistance is encountered and this change in the operating mode of the wrench may be utilised to automatically bring the marking device into operation.

The visual mark thus produced can be checked to see whether the correct amount of turn of the nut has been obtained and in the event that an insufficient turn has been applied it will be necessary to re-engage the wrench with the nut to apply the desired extra amount of turn. In order to ensure re-engagement of the marking device with the bolt shank at the correct position under such circumstances there is preferably used with the wrench a nut-engaging socket which is adapted for engagement with the wrench in one relative position only and in combination therewith a nut so formed as to have engagement with the said socket in one relative position only. This arrangement ensures that in the event of having to re-engage the wrench with the nut, for further rotation, the marking device will be correctly positioned at the end of the previously made mark on the bolt shank so that the further amount of rotation of the nut will appear as an extension of the previous mark 011 the bolt shank.

One form of socket for this purpose is shown in FIG- URE 3, such socket being in the form of a sleeve 42 of generally cylindrical form on the exterior and having, extending centrally therethrough, an opening which is of different cross-section at one end of the sleeve from the cross-section at one end of the sleeve from the crosssection at the other end.

The one end 43 of the sleeve constitutes the nut-engaging portion of the socket and the opening 44 at this end is of non-circular cross-section having a cross-section corresponding to a hexagon, but with one corner of the hexagon cut off (at 45) so as to provide an irregular seven sided figure.

The nut for use in combination with such socket would be similar to a conventional hexagon nut, but with one of its corners cut off so that it is only possible to engage the opening 14 in the socket with such nut in one position of the nut relative to the socket.

At the other end 46 of the sleeve the opening 47 is also of non-circular cross-section, having a cross-section which is square but with one corner of the square cut away (at 48) to provide an irregular five-sided figure. When this socket is used with the wrench shown in FIGURES 1 and 2 the portion 22 of the anvil member of the wrench will have a corresponding cross-section, one corner being cut away at 49, so that this opening 4-7 in the socket can only be engaged with the member 22 in one position of the socket relative to the member 22.

With this arrangement therefore, the socket has only one relative angular position upon the wrench and the socket is engageable with the nut in one relative position only, so that if a nut has been partially tightened the wrench can be re-engaged with such nut, with the guarantee that the marking tool will be accurately positioned 6 to re-cornmence its marking operation from the end of the previously made mark.

What I claim then is:

1. Apparatus for use in tightening a nut upon a bolt to achieve a predetermined tension in the bolt shank comprising an impact wrench in which a motor applies torque to a nut-engaging socket though a disengageable drive which includes a hammer device connected to the motor and an anvil device connected to the socket, and in combination therewith, a marking tool connected to the anvil device for rotation therewith and slidable axially in relation to the socket and means for moving said marking tool into engagement with the shank of the bolt, said means being operated by relative movement between the hammer device and the anvil device when the wrench is impacting.

2. Apparatus according to claim 1 wherein the wrench is pneumatically operated and comprising a cylinder formed in the anvil device, a piston working in said cylinder and connected to the marking tool and a valve member controlling admission of air under pressure to said cylinder, said valve member being connected with the hammer device so as to be opened as the hammer device retracts from the anvil device when thrown out of engageent therewith.

3. Apparatus according to claim 2 comprising an enclosing casing, a hammer device in the form of a sleeve rotatably and slidably mounted therein, means for rotating said sleeve, a plurality of driving dogs projecting from the forward end of said sleeve, an anvil device rotatably mounted in said casing, a plurality of driven dogs formed on the rearward end of the anvil device in opposition to said driving dogs, a nut socket mounting formed on the forward end of the anvil device, a passage extending axially through said anvil device, a marking tool slidably mounted in said passage and having its marking end projecting from the forward end thereof, a piston on the rearward end of said marking tool working in a cylinder formed in said passage, a valve seating in the rearward end of said cylinder, a. valve member controlling an air passage through said seating and a coil spring having one end engaging said valve member and the other end engaging the forward end of the hammer device.

4. In combination with apparatus according to claim 1, a nut-engaging socket member comprising a sleeve having an axially extending passage at each end, the passage at one end being engageable with the forward end of the anvil member of the impact wrench in non-rotative manner and the passage at the other end being of a noncircular cross-section which is such as to permit of engagement non-rotatively with a nut of corresponding crosssection in one position only.

5. A socket member according to claim 4 wherein the passage which engages with the anvil member has a cross section such as to permit of engagement with the anvil member in one position only.

References Cited UNITED STATES PATENTS 3,235,953 2/1966 Adams 29-407 3,255,521 6/1966 Callahan 29-407 1,131,225 3/1915 Farkell 81-523 2,622,465 12/1952 Kanter 8152.3 1,985,916 1/1935 Coates et a1. 73139 X 2,756,622 7/1956 La Belle 73-439 X 2,901,934 9/1959 Dunham 81-53 JAMES L. JONES, JR., Primary Examiner. 

